Apparatus and process for a polycondensation reaction
An improved process and apparatus for the production of a polyester or other condensation polymer is disclosed. In particular, polymerization is conducted in a reaction vessel equipped with a specially designed agitator that exposes the polymer melt partially filling the reaction vessel to inert gas flowing through the vessel. The agitator comprises a plurality of elements that lift a portion of a polymer melt in the reaction vessel and generate films of the polymer melt which films extends in planes that are parallel to the axis of the agitator and the flow of gas through the reaction vessel. In a preferred process, a melt of bis(2-hydroxyethyl) terephthalate, or its low molecular oligomers, obtained by esterifying terephthalic acid or transesterifying dimethyl terephthalate with ethylene glycol, is contacted with an inert gas at about atmospheric pressure in order to remove the reaction by-products and facilitate polymerization.
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Claims
1. A process for manufacturing a polyester of one or more aromatic dicarboxylic acids and one or more glycols in a molten state in which an inert gas is employed to assist in removing a volatile condensation by-product, wherein the improvement comprises, employing a horizontally disposed cylindrical reactor vessel partly filled with a polymerization reaction mass in the form of a melt, which reactor vessel is equipped with the following:
- a) a reactor inlet for introducing a polymerizable feed into the reactor vessel;
- b) a gas inlet for introducing an inert gas at or near one end of the reactor vessel and a gas outlet for removing the inert gas at or near an opposite end of the reactor vessel, thereby resulting in gas flow past the reaction mass in the reactor vessel;
- c) means for maintaining the reaction mass in the molten state;
- d) an agitator that rotates on its axis during operation, said agitator comprising a plurality of elements that are longitudinally disposed to convey a portion of the melt as said elements move through the reaction mass, the elements being positioned such that said elements generate films, the planes of the films being parallel to the central axis of the agitator and the flow of inert gas which is predominantly in the axial direction; and
- e) a reactor outlet for removing product polymer from the reactor vessel.
2. The process of claim 1 for the continuous production of higher molecular weight PET, the process conducted at atmospheric pressure or above comprising contacting DHET or its low molecular weight oligomers, in melt form, with an inert gas, flowing in the process countercurrently to the flow of the reaction melt, so that the ethylene glycol and other reaction by-products are removed continuously and wherein the product PET is removed continuously with a hold up time equal to less than about 5 hours.
3. The process of claim 1 wherein the process is conducted at atmospheric pressure.
4. The process of claim 1 wherein said surface area of the films is at least 30 square feet per cubic foot of melt.
5. The process of claim 1 wherein the elements are selected from the group consisting of elongated rectangular strips, screens, rods, wires, cut-out sheets, and punched-out sheets.
6. The process of claim 1 wherein the agitator additionally comprises wipers to wipe one or more internal walls of the reactor vessel.
7. The process of claim 1 wherein the agitator elements are spaced in a circular or polygonal geometry, in cross-section, at one or more radial distances from the axis of the agitator to form concentric cages.
8. The process of claim 1 wherein the polyester is poly(ethylene terephthalate).
9. The process of claim 1 wherein the inert gas is selected from the group consisting of nitrogen, carbon dioxide, and combinations thereof.
10. The process of claim 1 wherein the agitator is rotated at 2 to 12 rpm.
11. The process of claim 1 further comprising baffles.
12. The process of claim 11 wherein the baffles comprise donuts and discs, longitudinally spaced along the reactor vessel.
13. The process of claim 1 wherein the distance between the agitator elements is increased along the length from the feed end to the product end of the reactor.
14. The process as recited in claim 1 wherein the agitator spans at least about 75 percent of the internal length of the reactor vessel.
15. The process of claim 2 wherein the inert gas flows at a velocity of 0.2 to 3 ft/sec and an interfacial gas-liquid surface area is at least about 20 ft.sup.2 /ft.sup.3 of the melt.
16. The process of claim 2 wherein the agitator elements are 1/2 to 1 inch apart near the feed end of the reactor, the spacing is increased stepwise along the length of the agitator and the elements are 1.5 to 4 inches apart near the product end of the reactor.
17. The process of claim 2 wherein the polymerizable feed is an oligomer of bis(2-hydroxyethyl) terephthalate having a degree of polymerization of at least about 5 and polymerization to the final product degree of polymerization is conducted by contacting the reaction melt with the inert gas in a single reaction vessel.
18. The process of claim 2 wherein the inert gas is selected from the group consisting of N.sub.2 and CO.sub.2.
19. The process of claim 2 wherein the inert gas is preheated to about polymerization temperature or above polymerization temperature prior to contacting it with the melt.
20. The process of claim 1 wherein the polyester is poly(ethylene terephthalate), poly(1,3-propylene terephthalate), poly(1,4-butylene terephthalate), poly(ethylene 2,6-napthoate), or copoly(ethylene isophthalate/terephthalate).
21. The process of claim 2 wherein the temperature of polymerization is about 270.degree. C. to about 300.degree. C.
22. An apparatus for conducting a condensation polymerization in a molten state, which apparatus comprises a horizontally disposed cylindrical reactor equipped with the following:
- a) a reactor inlet for introducing a polymerizable feed into the reactor vessel;
- b) a gas inlet for introducing an inert gas at or near one end of the reactor vessel and a gas outlet for removing the inert gas at or near an opposite end of the reactor vessel, thereby resulting in gas flow past the reaction mass in the reactor vessel;
- c) means for maintaining the reaction mass in the molten state;
- d) an agitator that is adapted to rotate on its axis during operation, said agitator comprising a plurality of elements that are longitudinally disposed adapted to convey a portion of a melt at the bottom of the reactor as said elements move through the melt, the elements being positioned so that said elements are adapted to generate films, the planes of the films being parallel to the longitudinal axis of the reactor vessel; wherein the films that are thus generated provide most of the interfacial surface area in the reactor, and
- e) a reactor outlet for removing product polymer from the reactor vessel.
23. The apparatus of claim 21 wherein the agitator further comprises baffles along its length to improve inert gas distribution and/or reduce by-passing of the reaction mass in continuous operation.
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Type: Grant
Filed: Dec 23, 1996
Date of Patent: Jan 5, 1999
Assignee: E. I. du Pont de Nemours and Company (Wilmington, DE)
Inventor: Kamlesh Kumar Bhatia (Dewark, DE)
Primary Examiner: Terressa Mosley
Application Number: 8/771,494
International Classification: C08G 6400;